- A
The tunnel mode is multipoint, but the spokes need to be configured with 'tunnel mode gre ip' for direct communication.
Why wrong: DMVPN phase 2 uses multipoint GRE tunnels on spokes as well.
- B
The hub is missing the 'ipv6 nhrp redirect' command, and the spokes are missing 'ipv6 nhrp shortcut'.
Without redirect and shortcut, spokes do not learn each other's NHRP mappings and send traffic through the hub.
- C
The spokes have different NHRP network IDs, preventing registration.
Why wrong: The show output indicates both spokes are registered, so network IDs match.
- D
The IPv6 addresses on the tunnel interfaces are in different subnets.
Why wrong: All tunnel interfaces should be in the same subnet for DMVPN to work.
Question 2,112 of 2,152
300-410 IPv6 First Hop Security Practice Question
This 300-410 practice question tests your understanding of ipv6 first hop security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer is troubleshooting IPv6 DMVPN phase 2 spoke-to-spoke tunnel failures. Spoke routers are able to communicate with the hub, but direct spoke-to-spoke traffic is not working. Router R1 (spoke) has the following relevant configuration:
interface Tunnel0
ipv6 address 2001:DB8:1::1/64 tunnel source GigabitEthernet0/0 tunnel mode gre multipoint ipv6 nhrp network-id 1 ipv6 nhrp nhs 2001:DB8:1::2 ipv6 nhrp map multicast dynamic !
Router R2 (hub) shows: show ipv6 nhrp brief output indicates that both spokes are registered. What is the root cause?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The hub is missing the 'ipv6 nhrp redirect' command, and the spokes are missing 'ipv6 nhrp shortcut'.
In DMVPN Phase 2, spoke-to-spoke traffic requires the hub to send NHRP redirect messages and the spokes to use NHRP shortcuts. Without 'ipv6 nhrp redirect' on the hub and 'ipv6 nhrp shortcut' on the spokes, the spokes will forward all inter-spoke traffic through the hub instead of establishing a direct tunnel. The hub's NHRP brief shows both spokes are registered, confirming NHRP registration works, but the missing redirect/shortcut mechanism prevents direct spoke-to-spoke communication.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The tunnel mode is multipoint, but the spokes need to be configured with 'tunnel mode gre ip' for direct communication.
Why it's wrong here
DMVPN phase 2 uses multipoint GRE tunnels on spokes as well.
- ✓
The hub is missing the 'ipv6 nhrp redirect' command, and the spokes are missing 'ipv6 nhrp shortcut'.
Why this is correct
Without redirect and shortcut, spokes do not learn each other's NHRP mappings and send traffic through the hub.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The spokes have different NHRP network IDs, preventing registration.
Why it's wrong here
The show output indicates both spokes are registered, so network IDs match.
- ✗
The IPv6 addresses on the tunnel interfaces are in different subnets.
Why it's wrong here
All tunnel interfaces should be in the same subnet for DMVPN to work.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between DMVPN phases, and the trap here is that candidates assume NHRP registration alone enables spoke-to-spoke communication, overlooking the mandatory redirect/shortcut commands for Phase 2.
Trap categories for this question
Command / output trap
The show output indicates both spokes are registered, so network IDs match.
Detailed technical explanation
How to think about this question
DMVPN Phase 2 uses NHRP redirect (RFC 2332) where the hub sends redirect messages to inform a spoke of a more direct path to another spoke. The spoke then sends an NHRP resolution request to the target spoke to build a shortcut tunnel. Without 'ipv6 nhrp shortcut' on the spoke, the spoke ignores redirect messages and continues to forward traffic via the hub, causing suboptimal routing. This differs from Phase 3, where spokes use NHRP shortcuts proactively without relying on redirects.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPv6 First Hop Security — This question tests IPv6 First Hop Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The hub is missing the 'ipv6 nhrp redirect' command, and the spokes are missing 'ipv6 nhrp shortcut'. — In DMVPN Phase 2, spoke-to-spoke traffic requires the hub to send NHRP redirect messages and the spokes to use NHRP shortcuts. Without 'ipv6 nhrp redirect' on the hub and 'ipv6 nhrp shortcut' on the spokes, the spokes will forward all inter-spoke traffic through the hub instead of establishing a direct tunnel. The hub's NHRP brief shows both spokes are registered, confirming NHRP registration works, but the missing redirect/shortcut mechanism prevents direct spoke-to-spoke communication.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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